In recent years flow sensing devices, particularly gas flow sensing devices, have been designed and built by the use of technology comparable to integrated circuit technology. The devices are manufactured into silicon structures and can be manufactured to very small dimensions. Many of these flow sensing devices utilize temperature responsive resistors that are mounted either adjacent to or directly upon the silicon area. As such, integrated circuit technology can be utilized in the fabrication of the necessary electrical circuits for these devices.
Due to their exceedingly small physical dimensions, these devices are capable of responding very rapidly as they have a very small mass. This small mass allows for the thermal changes to occur rapidly. For gas flow sensors based on thermal conductance, flow sensitivity depends on the difference between the ambient gas temperature and the heater temperature. The greater the difference between the heater temperature and the ambient temperature, the larger the signal for a given flow. If the heater utilized in this type of a device is controlled at a constant temperature, an increase in ambient temperature will result in a decrease in the difference between the heater and ambient temperatures.
For differential pressure or flow velocity sensing applications, constant differential temperatures between a heater and ambient results in a sensor response that increases with decreasing ambient temperature. In some applications, this is undesirable, and means should be provided to compensate for the effects of ambient temperature.